Achievement

This study clearly highlights the critical role of electron irradiation in preventing the formation of bilayer loop edges in graphene materials. The understanding was achieved by using extensive ab initio quantum molecular dynamics simulations and quantum transport calculations enabled through large scale computing and demonstrates that vacancies and interstitials are key for keeping graphene layers parallel and preventing bi-layer edge coalescence (looping). The role of vacancies is to increase the surface reactivity and interlayer interactions far from the edges. Interstitials, on the other hand, provide effective feedstock for interlayer link creation that keep bi-layers parallel and prevents looping. Notably, it is the combination of vacancies and interstitials that keeps the edges open and avoid loop formation. Quantum transport calculations confirm that cross-linking of bi-layers increases the back scattering and creates significant transport between the layers. These cross-linking sites are key for the Joule heating defect cleansing and are susceptible to being healed during the process.

Significance

Graphene is a 2-Dimensional material with novel and outstanding electronic properties, which could revolutionize nanoelectronics in the near future. However, variations in thickness (stacked sheets) and edge shape or the presence of defects could significantly modify the electronic properties of graphene. It is therefore imperative to control both the number of stacked layers (thickness) and in particular the edge geometry, in order to optimize the electronic properties and in turn facilitate applied nanodevice design. Recent experimental results indicate that Joule heating can atomically sharpen the edges of CVD grown graphitic nanoribbons. However, thermal annealing (either by Joule or furnace) typically causes graphene layers to coalesce to form loops at the edges. The absence/presence of loops between adjacent layers in the annealed materials is the topic of considerable interest and debate that the present work has helped to clarify/understand. A rationale explaining why loops do form if Joule heating is used alone, and why adjacent nanoribbon layers do not coalesce when Joule heating is applied after high-energy electrons first irradiate the sample. Our work, based on large-scale quantum molecular dynamics and electronic transport calculations, shows that vacancies on adjacent graphene sheets, created by electron irradiation, inhibit the formation of edge loops.

Credit – This work was published in PRL 105, 045501 (2010). A portion of the research, performed at Oak Ridge National Laboratory's Center for Nanophase Materials Sciences, was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.